AOPA Turbine Pilot Magazine - March 2011 - 100

DOGFIGHT | RUN THE TANK DRY
Editor at Large Tom Horne and Senior Editor Dave Hirschman have a lot of things in common: lots of ratings, lots of experience in lots of airplane models—and lots of opinions (as well as similar haircuts). We last turned them loose on the topic of pattern entry (see “Dogfight: Pattern Entry,” January 2011 AOPA Pilot) and the response to two different schools of thought on this topic garnered some of our most interesting musings—and the largest amount of letters—from a large number of readers (see “Letters: Dogfight,” page 20). So we launch them into another “Dogfight”—we hope you’ll enjoy these two takes on a topic, and keep those cards and letters coming. —Ed.

Running on empty, momentarily
There are times when it’s a smart move
BY THOMAS A. HORNE >

s a general rule, it’s best to steer well clear of running a fuel tank dry. There are good reasons for this. Air drawn into the engine(s) may prolong a quick restart, for example. In addition, sediment or water that may be resting in the fuel-tank sumps can be sucked into the fuel system plumbing, causing rough engine operations. And yes, there’s the anxiety factor involved with any loss of power. But there are times when running a tank dry can be justified. Let’s say you are flying a long leg over the desert Southwest. Fuel availability is scarce over terrain like this, and when your exact fuel consumption rate is an unknown—even with your best efforts at flight planning—then running a tank dry may be the best way to confirm your fuel status and thus be able to plan a timely diversion, or perform a 180-degree turn. Of course, much depends on your airplane’s performance and equipment. In an airplane capable of flying 600 nm, there’s no reason to run a tank dry on a trip of, say, 300 nm. But if the same airplane was to attempt a 500-nm leg, then it may be advisable. Unforeseen strong headwinds—but not so strong that the airplane’s endurance is challenged beyond legal limits—may call for knowing exactly how much fuel remains aboard. Even ferry pilots with ferry fuel pay extremely close attention to fuel quantity. On several long over-water legs in Cessna 172s, I had the standard fuel aboard, plus a 50-gallon tank in the backseat, plus another 15 gallons in the baggage comAOPA PILOT •

It usually takes only a few seconds to restore fuel flow after a tank has run dry.
partment. Transparent fuel lines traveled from the tanks, down the center of the airplane (where I could view the flowing fuel) and through the firewall to the engine. After leveling off after takeoff, the drill was to switch to the aft tank—and watch the fuel line. When that first slug of air made its way backwards up the fuel line, I knew that the engine wouldn’t be running for long. As soon as the engine began to surge from fuel starvation I noted the elapsed time for the tank. That gave me an exact fuel-consumption rate. It usually takes only a few seconds to restore fuel flow after a tank has run dry. You can switch to a fuller tank and hit the fuel boost pump in a remarkably short period of time. True, fuel-injected engines may continue to surge for a bit, but it usually takes just 10 to 20 seconds or so for the engine to resume smooth operation. This is not a time for the faint of constitution, so running a tank dry is something you’d want to avoid with passengers. I prefer to do it when I’m flying solo. That procedure I described involved an airplane without a fuel-flow gauge, engine analyzer, or accurate fuel-measurement system. Pilots with this equipment have an advantage—and probably won’t ever have to run a tank dry in order to know their exact fuel condition. Most GA aircraft don’t have fancy aftermarket fuel-management gear. Their only fuel gauges are the notoriously unreliable fuel quantity indicators that look as though they’d be more at home on a lawn tractor than an airplane. Factor in potential inaccuracies in manifold pressure, rpm, and originalequipment fuel-flow indicators, and it’s easy to see how actual fuel burns—and ranges and endurances—can vary widely from those published in the airplane flight manual. Running a tank dry may be the only way to get a handle on what’s really happening under the cowl. Run a tank dry? Plan to avoid it. But understand when it may make sense. Better to know a tank’s running dry than to be surprised by complete fuel starvation. E-mail the author at tom.horne@aopa. org.

100 • MARCH

2011

AOPA Turbine Pilot Magazine - March 2011

Table of Contents for the Digital Edition of AOPA Turbine Pilot Magazine - March 2011